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Solar Window Calculator

Enter your latitude, month, panel orientation, and horizon obstructions — get sunrise, sunset, effective solar window, peak production hours, and seasonal June vs December comparison.

Location and panel orientation
°N
°
°
Horizon obstructions
°
°
Solar window — June
14.3 hr effective solar window — 9.1 hr peak production (sun >30°)
Sunrise04:51
Sunset19:09
Solar noon12:00
Total daylight hours14.3 hrs
Effective solar window (after obstructions)14.3 hrs (04:51 – 19:09)
Peak production hours (sun >30° altitude)9.1 hrs
Sun altitude at solar noon78.1°
Optimal panel tilt for annual production35° (your latitude)
Panel azimuth assessmentGood (southward)
Seasonal comparison
June — effective solar window14.3 hrs | 9.1 hr peak | noon altitude 78.1°
December — effective solar window9.7 hrs | 2.1 hr peak | noon altitude 32.0°
✓ Good solar window. Ensure shading analysis at spring/fall equinox when sun path shifts.
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How to Use This Calculator

Enter your latitude and select a month

Your latitude determines the sun's path across the sky. US latitudes range from about 25° (Miami) to 48° (Seattle). Find your latitude by right-clicking your location in Google Maps. Select the month you want to analyze — the calculator also automatically shows June vs December for seasonal comparison. December is the worst-case month for most US locations; June is the best.

Enter panel tilt and azimuth

Tilt angle is measured from horizontal — a flat roof is 0°, a vertical wall is 90°. The optimal tilt for annual energy production equals your latitude. Azimuth is the compass direction your panels face: 180° = true south (ideal in the northern hemisphere), 90° = east, 270° = west. Note: use true south, not magnetic south — there's up to a 20° difference in some US locations.

Enter horizon obstruction angles

Look east and west from your panel location. Estimate the angle above the horizontal where obstructions begin. You can use a clinometer app on your phone, a solar pathfinder tool, or simply estimate: a roofline at 20ft distance from 8ft height = roughly 15°. Buildings, trees, and hills all delay morning and cut off afternoon solar production.

The Formula

Solar Declination (δ) = 23.45° × sin(360/365 × (DayOfYear − 81)) Sunrise Hour Angle = arccos(−tan(Lat) × tan(δ)) Daylight Hours = 2 × Sunrise Hour Angle ÷ 15 Noon Altitude = 90° − Latitude + Declination Obstruction Clear Time = arccos((sin(ObsAngle) − sin(Lat)×sin(δ)) ÷ (cos(Lat)×cos(δ))) Effective Window = (West Clear HA − East Clear HA) ÷ 15 hours Peak Production Hours = time when sun altitude > 30° within effective window

Solar times are in solar time (solar noon = 12:00 by definition), which differs from clock time by your longitude offset within your time zone and the equation of time (up to ±16 minutes). For a precise clock-time conversion, add your longitude-based offset: (Standard Meridian − Your Longitude) × 4 minutes per degree.

Example

Urban canyon — Philadelphia, March, trees and buildings blocking horizon

A homeowner in Philadelphia (latitude 40°N) wants to install panels on a flat-roof addition. There's a neighboring building to the east blocking to 35° and mature trees to the west blocking to 35°.

Latitude40°N (Philadelphia)
MonthMarch (equinox)
Panel tilt40° (optimal)
Azimuth180° (true south)
East obstruction35°
West obstruction35°

Result

Total daylight12.0 hours
Effective solar window~4.5 hours (obstructions cut 7.5 hrs)
Peak hours (sun >30°)~2.8 hours
Solar noon altitude50.4°
June window~6.2 hours (sun higher, clears obstructions earlier)
December window~2.1 hours (low sun barely clears obstructions)

The 35° obstructions on both sides severely limit this site. In December, the sun barely clears 35° altitude even at noon in Philadelphia — the solar window shrinks to just 2 hours. The calculator correctly warns this site is marginal. The homeowner should consider a higher rack mount to clear the building shadow, or evaluate a different roof face with fewer obstructions.

FAQ

Solar time is based on the actual position of the sun — solar noon is when the sun reaches its highest point, which is exactly 12:00 in solar time by definition. Clock time is standardized by time zones, so solar noon occurs at different clock times depending on your longitude within the zone. In the eastern part of a time zone, solar noon occurs before 12:00pm; in the western part, after 12:00pm. The difference is (Standard Meridian − Your Longitude) × 4 minutes/degree. Additionally, the equation of time adds ±16 minutes of correction depending on the date. For site assessment purposes, solar time gives directly comparable results regardless of time zone.
Several methods work: (1) Clinometer app — free smartphone apps measure elevation angle precisely. Point at the top of the obstruction and read the angle. (2) Solar pathfinder — a physical tool that photographs the entire horizon in a curved dome, overlaid with sun path lines. Professional installers use these. (3) Manual estimation — a rough rule: if an obstruction is H feet tall and D feet away, angle ≈ arctan(H/D). A 30ft tree 100ft away is roughly 17°. (4) Aurora Solar or HelioScope — professional software that uses satellite 3D models to calculate shading automatically.
The sun's altitude at a given time varies significantly by season — up to 47° between summer solstice and winter solstice. An obstruction that causes minimal shading in June (sun is high) can block virtually all solar production in December (sun is low). Trees without leaves in winter may cast less shade than in summer, partially offsetting the low sun angle. For annual production estimates, you need both June (best case) and December (worst case). Also check equinox months (March, September) when the sun path shifts, as obstructions that don't affect summer or winter may shade the equinox path.
Most solar professionals consider 4+ hours of effective solar window as the minimum for a viable installation. Below 4 hours, production is significantly reduced and payback periods extend substantially. The key metric isn't just total hours but peak production hours — hours when the sun is above 30° altitude. Below 30°, panel output drops steeply due to the long atmospheric path length (air mass). A site with 6 hours of solar window but only 2 peak hours (low sun angles) will produce less than a site with 5 hours including 4 peak hours.
Azimuth matters significantly at high obstructions and high latitudes. True south (180°) is optimal — it maximizes the time the sun is in front of the panel. East-facing panels (90°) get morning sun only; west-facing (270°) get afternoon sun only. Either orientation reduces annual production by roughly 15-20% vs south-facing at mid-latitudes. However, west-facing panels produce more during TOU peak hours (4-9pm), which can be financially optimal in high-TOU rate markets despite the production reduction. Southeast (135°) and southwest (225°) each lose about 5-8% vs true south and are generally acceptable.

Related Calculators

☀️ Solar Angle Calculator
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💡 Solar Irradiance Calculator
Calculate peak sun hours and irradiance by location and month.
☃ Solar Panel Calculator
Size your full solar system based on annual kWh and location.
🌳 Solar Shading Calculator
Estimate production loss from roof obstructions and shading.

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  width="100%" height="750" frameborder="0"
  title="Solar Window Calculator"></iframe>